Catalytic cracking processes have been developed principally for upgrading feedstock derived from natural or synthetic crude oil to more valuable hydrocarbon mixtures, particularly of lower molecular weight. These lower molecular weight hydrocarbons are generally more desirable because of their higher quality and market value. In a typical catalytic cracking process, a crude oil derived feedstock is contacted with a hot, regenerated catalyst, at temperatures ranging from about 1200° F. to about 1400° F. and low to moderate pressures. The chemical reactions that take place in the presence of the catalyst include predominantly scission of carbon-to-carbon bonds (simply cracking) isomerization, polymerization, dehydrogenation, hydrogen transfer, and others, generally leading to lower molecular weight hydrocarbon products.
Some of the cracking reactions in the catalytic cracker also produce hydrocarbonaceous compounds of high molecular weight, of very low volatility, of very high carbon content and of low combined hydrogen content. The hydrocarbonaceous compounds tend to be deposited on the active surfaces of the cracking catalyst and mask the active sites, rendering the catalyst less active, thus unsuitable for continued cracking without regeneration. Deposits of the hydrocarbonaceous matter and the inclusion of absorbed and adsorbed hydrocarbons, as well as the vaporous combustible components in the fluidizing media between the solid catalyst particles, collectively called “coke,” are in a sense undesirable. In response to the undesirable buildup of coke on the surfaces of the catalyst, the oil and gas industry has developed several techniques to reduce or remove such buildups.
One technique currently used to reduce the coke forming characteristics of feedstocks, includes without limitation, hydrotreatment, distillation, or extraction of the natural or synthetic crude feedstock prior to charging it to the catalytic cracker. Hydrotreatment, distillation, or extraction of the crude oil derived feedstock serves to remove a substantial amount of the coke precursors, such as contained in asphaltenes, polynuclear aromatics, etc., prior to catalytic cracking. Hydrotreatment, distillation, or extraction are somewhat effective in reducing or removing large amounts of coke precursors from the crude oil derived feedstock, however, such processes are expensive and time-consuming. Currently, incrementally available crude oil is of high residuum content and of higher coke forming characteristics at a time when it is unpopular or unlawful to utilize this additional residuum as fuel oil. At the same time, the market for residuum products, other than as fuel oil, is saturated. Additionally, to upgrade the residuum materials by the available technology results in products of lower quality (and lower market value) than would be achieved by catalytic cracking, provided the coke yield can be handled. Moreover, current and anticipated Federal and State legislation has, and is, scrutinizing the environmental and storage issues associated with use, removal or conversion of the coke precursors. Therefore, there is a great need for an environmentally responsible conversion of the residuum portion of crude oil.
Another technique currently used to remove coke formation from the spent cracking catalyst is to burn the coke away from the catalyst surface using an oxygen-containing gas stream in a separate regeneration reactor. In such a situation, air, oxygen, carbon dioxide, and steam for diluent as combustion gas, may be introduced into the spent cracking catalyst in the lower portion of the regeneration zone(s), while cyclones are provided in the upper portion of the regeneration zone for separating the combustion gas from the entrained catalyst particles. The coke buildup removal process attempts to substantially remove the coke buildup, and is generally effective, but large amounts of greenhouse gases are produced, at least some of which are released into the atmosphere, which is generally environmentally undesirable. Another technique teaches the use of a waste heat boiler as a means of reducing greenhouse gases going to the atmosphere, however, the reduction by this method remains limited to the achievable concentration of a fired heater. In each of the aforementioned techniques the initial regeneration zone is operated in a partial oxidizing (reducing) mode, and any other regeneration zone is operated in an oxidizing mode. U.S. Pat. No. 4,388,218 entitled “Regeneration of Cracking Catalyst in Two Successive Zones” to Rowe, and U.S. Pat. No. 4,331,533 entitled “Method and Apparatus for Cracking Residual Oils” to Dean et al., further detail such processes and are included herein by reference.
Similarly, the regeneration zone must be carried out in such a way that it is in thermal equilibrium with the cracking reaction zone. In other words, the sensible heat of the hot regenerated catalyst in the catalytic cracker should be in balance with the heat requirements of the catalytic cracking reactor zone. In conventional operations, excluding the use of internal or external cooling coils for removing heat from the regenerator reaction zone, coke yield of only about 5 to about 8 weight percent of the total feed may be burned from the catalyst, without exceeding the amount of heat required to balance and sustain the cracking reaction.
Thus, to maintain the thermal balance needed to operate the catalytic cracker and remove enough of the coke from the catalyst to sustain the cracking process, one of two things should be done. First, the amount of coke that forms on the surface of the catalyst should be reduced. However, as mentioned above, this can typically be accomplished by using higher quality feedstock, which is more costly, or subjecting the currently available feedstock to the previously mentioned upgrading, such as but not limited to, hydrotreatment, distillation or extraction processes, which are also more costly. Second, internal or external cooling units could be installed in the regeneration units. However, such internal or external cooling units are costly and unreliable.
Accordingly, what is needed in the art is a method of catalytically cracking crude oil derived feedstock having high coke forming characteristics, without experiencing the drawbacks of the prior art methods.